dt-bindings: spi: Adjust the bindings for the FSL QSPI driver

Adjust the documentation of the new SPI memory interface based
driver to reflect the new drivers settings.

The "old" driver was using the "fsl,qspi-has-second-chip" property to
select one of two dual chip setups (two chips on one bus or two chips
on separate buses). And it used the order in which the subnodes are
defined in the dt to select the CS, the chip is connected to.

Both methods are wrong and in fact the "reg" property should be used to
determine which bus and CS a chip is connected to. This also enables us
to use different setups than just single chip, or symmetric dual chip.

So the porting of the driver from the MTD to the SPI framework actually
enforces the use of the "reg" properties and makes
"fsl,qspi-has-second-chip" superfluous.

As all boards that have "fsl,qspi-has-second-chip" set, also have
correct "reg" properties, the removal of this property shouldn't lead to
any incompatibilities.

The only compatibility issues I can see are with imx6sx-sdb.dts and
imx6sx-sdb-reva.dts, which have their reg properties set incorrectly
(see explanation here: [2]), all other boards should stay compatible.

Also the "big-endian" flag was removed, as this setting is now selected
by the driver, depending on which SoC is in use.

[2] https://patchwork.ozlabs.org/patch/922817/#1925445



Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>